Containers on a suitable user-defined network can reach each other by configured service names. Localhost inside a container refers to that container.
Before you start
You should understand the difference between an image, a running container and the host. Record where a file, process or network endpoint actually lives before diagnosing a problem. Commands illustrate local experiments; adapt image names and paths to a disposable development environment.
The practical goal is to reason through this situation: An application connects to the database service name, not its own localhost. Read the walkthrough first, then try the interview exercise before opening its answer. The important part is explaining the decision and its consequences, rather than remembering a definition alone.
Step-by-step walkthrough
Step 1: Locate network boundaries
Localhost within an application container names that container.
Step 2: Use service identity
Connect to the database’s service name on a suitable shared network.
Step 3: Check listening and membership
DNS resolution alone does not prove the target accepts connections.
Worked scenario
An application connects to the database service name, not its own localhost.
An application attempting localhost:5432 searches inside itself rather than another container. On the intended user-defined network, the database service name supplies discovery. A database listening only on an unsuitable interface or containers attached to different networks can still prevent access.
Common mistake
Hard-coding transient container IPs breaks when instances are recreated.
Verify the behavior
Verify resolution, network membership, listening address and port before changing credentials.
Interview exercise
Troubleshoot a failed connection.
Answer and reasoning
Check network membership, service name, listening address and port before changing credentials.
Continue learning
Compare the scenario with the Docker interview questions and test your understanding with the Docker MCQs. For terminology and implementation details, consult the reference material.